Short answers, pulled from the story.
Constantius Chlorus was a Roman emperor who served as caesar from 293 and became senior augustus of the western empire in May 305, ruling until his death on the 25th of July 306. He was the father of Constantine the Great and the founder of the Constantinian dynasty, which governed the Roman Empire until the death of his grandson Julian in 363.
The name Chlorus was a coinage of Byzantine-era historians, popularized long after Constantius's death, and was never used during the emperor's lifetime. His formal name was Marcus Flavius Valerius Constantius.
Constantius Chlorus reconquered Britain from the usurper Allectus in 296 by launching two invasion fleets across the English Channel. His praetorian prefect Julius Asclepiodotus landed near the Isle of Wight and defeated Allectus, while Constantius occupied Londinium and killed Frankish mercenaries who were threatening the city. He also reorganized the British provinces and restored Hadrian's Wall.
Constantius Chlorus was formally appointed caesar by Maximian at Mediolanum on the 1st of March 293, making him one of the four original members of Diocletian's Tetrarchy. He ranked as the more senior of the two caesares and on official documents always took precedence over Galerius. In May 305, when Diocletian and Maximian abdicated, he became senior augustus of the western provinces.
Of the four Tetrarchs, Constantius Chlorus made the least effort to enforce the Diocletianic Persecution, which began in 303. He limited himself to demolishing a handful of churches and did not pursue, imprison, or require sacrifices from Christians. Ancient sources disagree even on that: Eusebius denied he destroyed any Christian buildings, while Lactantius recorded that he did.
Constantius Chlorus died on the 25th of July 306 at the Roman fortress of Eboracum, the city now called York. He had just completed a campaign against the Picts in northern Britain. As he lay dying, he named his son Constantine as his successor, and the legions at York immediately acclaimed Constantine as emperor.